Managing Apple Crop Load and Diseases with Bloom Thinning Applications in an Organically Managed ‘Honeycrisp’/‘MM.111’ Orchard

in HortScience

Although demand for organic produce continues to increase in the mid-Atlantic, few apple (Malus ×domestica Borkh.) growers in the region have adopted organic management practices due to the considerable disease, insect, and weed pressure, as well as the lack of effective crop load management tools. In this study, lime sulfur (LS) and Regalia® (R) were applied in different sequences (i.e., LS/LS, LS/R, R/R, and R/LS), each in a mixture with JMS Stylet-Oil, to chemically thin apple flowers in an organically managed ‘Honeycrisp’/‘MM.111’ orchard. There was also a nontreated control, a “grower standard” control (LS at 11 mm fruitlet diameter), and a hand-thinned control. The treatments were evaluated for their ability to reduce crop load, as well as to control powdery mildew [Podosphaera leucotricha (Ellis & Everh.) E. S. Salmon], cedar apple rust (Gymnosporangium juniperi-virginiana Schwein.), and quince rust (Gymnosporangium clavipes Cooke & Peck). All treatments reduced crop load compared with the nontreated control, and after the first application of LS or R, the number of fertilized king blooms was reduced and fertilization was prevented in all side blooms. All bloom thinning treatments had more fruit peel russet than the control and russet was more severe when LS was one of the applications. Bloom thinning applications of LS and R did not reduce powdery mildew leaf infection compared with the nontreated control. Cedar apple rust incidence was reduced by all bloom thinning treatments, though some lesions were detected in all treatments. There were minimal quince rust infections in any of the treatments, including the nontreated control. These results suggest that when LS and/or Regalia® are mixed with JMS Stylet-Oil and applied as bloom thinners, they can reduce crop load, and, as a secondary benefit, they can also decrease cedar apple rust incidence from infections that occur during bloom.

Contributor Notes

Funding support was provided by a U.S. Department of Agriculture-Specialty Crop Block Grant administered by the Virginia Department of Agriculture and Consumer Services, Virginia Agricultural Experiment Station, and Virginia Tech’s Department of Horticulture.

We thank Sierra Athey, Matt Borden, David Carbaugh, Allen Cochran II, Scott Kilmer, Abby Kowalski, Taylor Mackintosh, Jared Repass, William Royston, Jr., Ashley Thompson, and Jim Warren for their assistance with this experiment.

Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the authors and does not imply its approval to the exclusion of other products or vendors that also may be suitable.

Corresponding author. E-mail: gmp32@cornell.edu.

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Article Figures

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    Graphical representation of the ‘Honeycrisp’-specific apple pollen tube growth model from 2015. Hourly temperature data (dotted line) were collected from a weather station located near the experimental orchard and cumulative pollen tube growth (solid line) was calculated on an hourly basis. Average style length (dashed line) was 9.0 mm, as measured on 50 king bloom flowers on 20 Apr. Bloom thinning treatments were applied at 0800 hr on 27 Apr. and 1600 hr on 29 Apr. Cumulative pollen tube growth was reset to 0 mm after each application.

  • View in gallery

    The percent of king and side apple flowers that were fertilized 24 h after the first bloom thinning applications of lime sulfur or Regalia® (both applied with JMS Stylet-Oil) on 27 Apr. Fertilization was determined visually using fluorescence microscopy by determining whether pollen tubes reached the base of the style. Mean separation within king or side bloom by Tukey’s honestly significant difference (P = 0.05). Data were arcsine square root transformed before analysis, but means (n = 4) are shown untransformed.

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